A) General Concerns
PET containers subjected to hot fill applications must address two fundamental concerns which are not present in conventional PET container applications. The primary concern arises because liquid food products must be poured and sealed in a container at an elevated temperature high enough to destroy bacteria, microorganisms and the like to sustain food purity. The thin side walls of conventional PET containers thermally distort or collapse at hot fill temperatures. Assuming that a PET container can be formed in a configuration which will maintain its shape at the hot fill temperature, i.e. "thermally stable", the container is then subjected to a vacuum which is inherently drawn within the sealed or capped container when hot food products cool and contract. The container has to either withstand the vacuum or collapse in a shape retaining manner. For this reason and until relatively recently, rigid containers, such as glass, have been used in hot food applications.
From theoretical considerations, it is known that thermal stability of a PET container can be increased if the crystallinity of the material is increased. It is also known that thermal crystallization of amorphous PET is a time-temperature relationship and that, given a sufficient time period at elevated temperatures, amorphous PET will develop spherulitic crystallization which is opaque and brittle. It is also known if the molecular PET string is aligned or molecularly oriented by stretch blowing at preferred temperatures, crystallinity of the molecularly oriented PET can also be increased at increased temperature, but the crystallite structure will not be opaque nor excessively brittle. These concepts are the basis for all processes which produce PET containers for hot fill applications.
B) Thermally Stabilized, PET Prior Art
Until the present invention, hot fill applications using PET containers have been commercially practiced in accordance with one of two processes. The first process, developed by Monsanto and commercially produced today by Johnson's Control is basically disclosed in Agrawal U.S. Pat. No. 4,497,855. In the Monsanto process, a PET preform is heated to its preferred orientation temperature which is generally within the range of 195.degree. F. to about 205.degree. F. and the preform is blow molded into the desired container configuration in the normal manner. As Agrawal notes, blow molding of a PET preform raised to its preferred orientation temperature in a cold mold will produce a container having a crystallinity of about 20 to about 28%. To increase crystallinity, the container is transferred to a hot mold which is maintained at temperature ranges of anywhere from about 240.degree. F. to about 400.degree. F. and preferably from about 240.degree. F. to about 270.degree. F. and at this temperature, held for a sufficient time period, the crystallinity increases to a range of about 28 % to about 32%. At that crystallinity range, Agrawal teaches that a container with acceptable thermal distortion, i.e. less than about 1% for hot fill liquids at temperatures of up to about 185.degree. F. will result and that the bottle will be clear. This "heat treatment" step will hereafter be referred to herein as "heat set". As noted, PET containers produced in accordance with the Monsanto or Johnson Controls process are acceptable but the process requires a separate heat set step.
There are, of course, numerous variations of the heat set step in the literature. For example, U.S. Pat. No. 4,385,089 to Bonnebat discloses a partial heat step in which preform is blown at its preferred molecular orient temperature into contact with a cold blow mold which blow mold is then raised in temperature of about 40.degree. C. to effect a partial thermofixing of the heated material after which the mold is cooled and the article removed.
An alternative process which has also gained commercial acceptance is marketed by Continental PET Technologies, Inc. and is discussed in the literature in Collette U.S. Pat. No. 4,618,515; Karins U.S. Pat. No. 4,665,682 and Beck U.S. Pat. No. 4,496,064. What is disclosed in the aforementioned patents is a thermally stable container having, generally speaking, a wide mouth so that the mouth or the neck portion of the container, as well as the body portion, is molecularly oriented under conditions which produce sufficiently high crystallinity percentages to be thermally stable. This avoids the heat set step. In the listed patents, the neck or the mouth of the container has to be molecularly or biaxially orientated to eliminate amorphous regions and in order for this to be accomplished an intermediate article has to be first formed with the amorphous end of the preform subsequently discarded. When the mouth of the container is of such a size that it cannot have a high degree of biaxial orientation, Collette then teaches to increase the temperature of the preform at the mouth to a point whereat spherulitic crystallization occurs to achieve a high enough crystallinity level to avoid temperature deformation. The spherulitic crystallization, of course, produces opacity. Thus, Continental's process, especially suitable for wide mouth containers, eliminates a heat set step but produces an intermediate formed article which requires a portion of the preform to be severed from the container and re-melted.
Thus, both processes involve additional steps which are costly and time consuming. Further, both processes are somewhat limited when the hot fill application requires a container with a small threaded mouth. Such containers are typically produced by preforms where the neck or mouth finish is formed in the open end of the preform and the neck finish is not distended in the blowing stage when the container is formed. Thus, the neck finish is not molecularly oriented. Should the neck finish be subjected to particularly high temperatures, such as that in the heat set step, and if the finish is not rapidly cooled, spherulitic crystallinity will produce whitening or clouding of the finish whereas the body portion of the container which has been molecularly oriented and quickly cooled will not experience whitening or clouding at the heat set temperature specified. Accordingly then, provisions must be made in the heat set process to rapidly cool the neck finish and then increase wall thickness of the neck finish if a clear container is desired, or alternatively, to produce spherulitic crystallization in the essentially amorphous neck finish as discussed.
Finally, both processes use a two stage process to produce the preform. Heretofore, a one stage process has not been used to produce PET containers suitable for hot fill application.
C) Single Stage PET Processes
In a two stage process, a preform previously made and substantially in an amorphous state is reheated to its preferred molecular orient temperature whereat it is blow molded into its desired shape. As used herein a two stage process will mean any process which produces a preform that is required to be reheated from ambient temperature to its blow molding temperature. In contrast, single stage processes form the preform, transfer the preform from the injection or extrusion mold (after it has cooled to some temperature) to a conditioning or tempering station whereat it homogenizes or equilibriates to a preferred molecular orient temperature. The preform is then transferred to a blow mold whereat it is formed into its desired shape. This process is generally illustrated in assignee's Stroup U.S. Pat. No. 4,372,910 and the preferred embodiment of the present invention has been practiced in a machine (Van Dorn Plastic Machinery's CIB single-stage injection/blow molder) generally disclosed in Stroup.
Inherent in any one stage process, is the fact that an unequal distribution of heat will exist through the cross-sectional wall thickness of the preform at the time it is transferred from the injection or extrusion mold. A number of patented processes exist relating to the timing and temperatures of the preform when it is pulled from the injection mold to optimize the cycle time. Ryder U.S. Pat. No. 4,473,515 discusses a number of such patents and discloses a process which waits until the average wall temperature in the injection mold reaches the preferred molecular orient temperature before the preform is transferred so that the preform simply can equilibriate to a uniform cross-sectional wall temperature equal to the preferred molecular orient temperature. In Valyi U.S. Pat. No. 4,382,905 an "interrupted quench" is disclosed in that the preform is removed from the injection mold "prematurely" and the cooling thereof continued in a conditioning mold after which the preform is transferred to yet another conditioning or temper station at a higher temperature whereat it equilibriates to its preferred molecular orient temperature, etc. In Valyi, injection time is thus shortened to increase utilization of the injection molds. All of the single stage patent literature discloses the final shaping or molding of the container with PET material which is somehow brought through a conditioning station to a uniform cross-sectional wall temperature which is at a preferred molecular orient temperature for the PET.
It is well known that molecular orientation of PET may occur over a temperature range varying from just above the glass transition temperature, T.sub.g (which is the temperature below which the PET is in a glassy state), up to just below the melt temperature of PET. If orientation is attempted at a temperature too close to the T.sub.g temperature, the material is too stiff to stretch. As the temperature increases, processing improves significantly but a practical upper limit is reached at or near the temperature at which large aggregates of crystallites called spherulites begin to form. The orientation process is adversely affected by spherulite growth. The preferred molecular orient temperature for conventional PET as taught in the patents discussed is universally known to be within a close range either between 190.degree.-200.degree. F. or 195.degree.-205.degree. F. The single stage processes in accordance with the literature noted above, will not produce PET containers which can withstand deformation at elevated temperatures because blow molding the containers at the preferred molecular orient temperature will not produce a crystallinity in the structure sufficiently high to withstand deformation.
D) Prior Art Container Configurations
Generally speaking, the container deformation problems attributed to vacuum are treated separately from the thermal deformation problem discussed above although crystallinity increase may have some effect on container rigidity.
The Agrawal reference discloses the concept of a vacuum deformable panel which is molded into the container. The panels are surface indentations formed in the container's side walls and/or in the container's base which pull inwardly under a vacuum. Of course, when the cap is opened, the panels expand radially outwardly to their molded shape. By positioning the panels, bottle or container shape may be kept within some degree of overall configuration.
Various configurations have been developed other than panels to maintain the overall shape of the container. In Ester U.S. Pat. No. 4,610,366, sharply formed indentations are provided in the side wall to maintain the bottle diameter while permitting vertical movement. The ring configuration thus disclosed in Estes permits the bottle to move as an accordion while retaining its O.D. dimension.
While such arrangements generally perform an acceptable function of overall shape retention, the bottles all flex their side wall. This in turn requires special labels and/or labelling techniques to be used by the bottler. In typical glass bottling operations, a light weight paper label is simply glued onto the bottle as it is rolled after the bottle is hot filled and capped. The PET side wall configurations of the prior art move and will not hold conventional, glued light weight paper labels, especially when the vacuum seal is broken and the bottle expands.
Apart from labelling concerns, conventional bottling operations, at least for small mouth bottles, typically employ star wheel layouts whereat bottles are filled in one wheel and transferred to another wheel whereat a capper screws the closure onto the bottle while the wheel rotates at high speed. It can be appreciated that the fill is still hot when the capping mechanism tightens the closure. The ability of PET material to withstand compressive forces is reduced at elevated temperatures. While the capper mechanism can be and has been altered to reduce axial force loadings on PET containers and/or PET containers have been altered to provide mouth rings for loading purposes, such factors affect the bottle configuration. Apart from any of these conditions, PET bottles must have a side wall strength sufficient to permit stacking one on top of the other without collapse. Vacuum panels indented into the side wall do not assist the container in developing sufficient axial strength to resist deformation.